feat: #3627507 Podcast, show and episode pages after the design, on the show-and-episodes model

Closes #3627507.

Builds the Podcasts listing, show and episode pages after the design, on the show-and-episodes model, with 6 demo shows and 67 episodes.

• Includes the commit from #3627367 (MR !59 (closed), now closed) unchanged.
• Depends on #3622954 (Varbase Podcasts Base, merged via !14 (merged)) and #3627494 (Vartheme BS5 Rightup, merged via !54 (merged)).

Verified on a fresh install from these branches: 6 shows and 67 published episodes, all three pages render, AJAX load more works, share icons match the design, and axe-core finds 0 WCAG 2.2 AA violations on each page.

Locally, Composer validate, YAML lint, Cspell, ESLint and Cucumber dry-run pass. The install and functional jobs could not run locally because ppa:ondrej/php (Launchpad) was unreachable from the dev host, so this MR's pipeline is the evidence for those.

AI-Generated: Yes

Checkpoints:

• File an issue
• Addition/Change/Update/Fix
• Testing to ensure no regression
• Automated unit testing coverage
• Automated functional testing coverage
• UX/UI designer responsibilities
• Readability
• Accessibility
• Performance
• Security
• Developer Documentation
• User Guide Documentation
• Reviewed by human
• Code review by maintainers
• Full testing and approval
• Credit contributors
• Review with the product owner
• Release notes snippet
• Release

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